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My transmission shifts really bad from 1st to 2nd (C-4 on 200 I6, 66 coupe). It takes it a second and a half or even two seconds to catch it. If I put thicker or thinner transmission fluid in, will it help? It's not the vaccuum modulator valve... I just put a new one on it and adjusted it just like the book said.
 

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Its possible your filter (or screen) is partially clogged causing the low pressures from the front pump. If not check your throttle valve linkage (passing gear) for binding. If that doesn't correct the problem you may have worn clutches or a band out of adjustment...
 

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I agree with Blindman. A change of tranny fluid and filter couldn't hurt. Another reason that the transmission might be slipping that is relatively easy to fix is replacement of the vacuum modulator at the back right side of your C4. The diaphram in these sometimes develops leaks and will cause sloppy shifting. The vacuum line comes down from the intake manifold to the modulator. You just remove the vacuum line from the modulator and use a wrench to remove the unit. A new one costs around $11. Check your transmission fluid for a burned smell when you drain it. That could indicate excessive wear of the clutch packs. While the fresh fluid, filter and modulator might remove the problem, the damage that may have been done to the clutch surfaces may mean a rebuild.
 
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